Hemodialysis treatment is employed as a therapeutic measure when a patient's kidneys no longer perform their blood purifying function because of disease or traumatic removal. Kidney failure results in the accumulation of toxic waste in the patient's blood and eventual death from uremic poisoning, unless the waste material is removed by some artificial means. In hemodialysis of the type to which the present invention relates, the patient's blood is circulated from the patient in a closed blood circuit by a pump to one side of a membrane contained within a hemodialyzer (i.e., artificial kidney). The membrane has pores of microscopic size through which waste products from the blood pass. The pores are, however, too small to permit blood cells and proteins to leave the body. A dialysis fluid (dialysate) is circulated on the other side of the hemodialyzer membrane to remove the waste products. The dialyzed blood is returned to the patient.
The blood pump normally used in contemporary dialysis systems is a roller pump in which rollers on rotating arms radiating from a motor-driven shaft progressively squeeze closed a section of flexible tubing in the circuit connecting the patient's blood system to the hemodialyzer. The speed of the pump motor is set to give a pump output in accordance with the anticipated patient's blood delivery rate. However, this delivery rate will normally vary during treatment and may drop below the pump output setting, a condition likely to collapse and obstruct the connection tubing. In contemporary dialysis machines an alarm is triggered and the blood pump is stopped if the patient's blood delivery rate drops below the pump flow rate setting. To minimize the occurrence of such a condition the blood pump is normally given a flow rate setting below the anticipated blood flow rate from the patient to allow for downward fluctuations of the patient's blood delivery rate. Hence, the treatment time is longer than would be necessary if the pumping rate continuously matched the patient's blood delivery rate.